The thinker that has done most to mount a defence of Christianity against Nietzsche's ferocious onslaught in On the Genealogy of Morals (OGM) – actually, not so much a defence as a counter attack – is the brilliant French sociologist René Girard. Girard critically examines Nietzsche's central contention that Christianity is a religion of sublimated vengeance or ressentiment and contents that although Nietzsche is half right about Christians he remains dangerously naive about violence itself.

Girard's main area of interest is in the relationship between religion and violence. His work looks at the ways in which violence often becomes self-perpetuating, one act of violence eliciting a mirrored response: thus the idea of an "eye for an eye and a tooth for a tooth". For Girard, the teachings of Christ are an attempt to break this wheel of revenge. Instead of the endless reciprocity of eye for an eye, forgiveness breaks the cycle, revenge is forsworn and violence not answered back in kind.

The day after 9/11, Rowan Williams, who was caught up in the attack on the world trade centre, was phoned up by a Welsh speaking journalist. In his book Writings in the Dust, he describes that his first reaction was to wonder in which language to respond to the journalist, a reflection that immediately turns to what language in which answer back the terrorists. Is violence really "the only language they understand" he asks, his thoughts turning back to ask about the role of forgiveness.

It is a classic piece of Girardian writing.

Forget, then, the idea that forgiveness is some sentimental means of the victim thinking well of those that have done them harm – often this is impossible. Rather, Christian forgiveness is much more practical and empirical: it's about not answering back in kind, not returning violence with more of the same. In essence, it represents a stubborn refusal to act in the same way as the violent other, it is a refusal to become like them.

This emphasis on forgiveness thus throws itself directly in the path of Nietzsche's charge of ressentiment. Because forgiveness refuses the satisfaction of vengeance it generates ressentiment. So Nietzsche is partly right. Yes, there are huge wells of anger that form within the Christian imagination. Yes, the instinct for vengeance is not spirited away by the Christian act of forgiveness. If you punch me and I choose to forgive and not to punch back, there will still be an emotional consequence of living with the lingering anger that has not been discharged in action or revenge.

Nonetheless, Girard argues, the very fact that Christians have chosen to forgive and thus not to answer violence directly with violence is itself already a huge victory. He puts it thus:

Ressentiment is the interiorisation of weakened vengeance. He [Nietzsche] sees ressentiment not merely as the child of Christianity, which it certainly is, but also as its father, which it certainly is not. Ressentiment flourishes in a world where real vengeance has been weakened. The Bible and the Gospels have diminished the violence and vengeance and turned it into ressentiment not because they originate in the latter but because their real target is vengeance in all its forms, and the succeeded in wounding vengeance not eliminating it. Ressentiment is the manner in which vengeance survives the impact of Christianity.

(from Nietzsche versus the Crucified)

In other words, Nietzsche is brilliant at diagnosing the hidden hatreds that lurk within the Christian breast, but he does not appreciate that these hatreds are themselves the by-product of a victory over real violence. Ressentiment is the collateral damage of forgiveness.

For all his philosophical machismo, Nietzsche was remarkably naive about the reality of violence. For him it was almost a game. Consider this telling account of how Nietzsche received his duelling scar from a university rival: "We had a very animated conversation about all things, artistic and literary and when we were saying goodbye, I asked him in the politest terms to duel with me." The fight was described thus:

It scarcely lasted three minutes, and Nietzsche's opponent managed to cut a low carte at the bridge of his nose, hitting the exact spot where his spectacles, pressing down too heavily, had left a red mark. Within two or three days out hero had recovered, except for a small slanting scar across the bridge of his nose which remained there throughout his life and did not look at all bad on him.

It was only because Nietzsche treated violence a bit like a game that he could think of violence as a cure for ressentiment. Girard puts it thus:

He did not see that the evil he was fighting was a relatively minor evil compared to the more violent forms of vengeance. He could afford the luxury of resenting ressentiment so much that it appeared a fate worse than real vengeance. Being absent from the scene real vengeance was never seriously apprehended.

The truth is, Christianity takes violence a good deal more seriously than Nietzsche himself, despite his fancy rhetoric and insightful analysis.

There is an important rider to all of this, however. For quite a lot of Christian theology has little place for forgiveness. The evangelical doctrine of penal substitution, for instance, argues that human beings are saved through a process whereby the violence that is due to human beings (because of human disobedience) is instead discharged upon Jesus: thus, the cross. He "pays the price of sin". This nasty and pernicious theology is built around the idea of a holy lynching and forgiveness plays little part. Of course, Jesus himself taught that religion ought to be reconstructed around the idea of forgiveness rather than blood sacrifice. Even so, penal substitution simply perpetuates the grim ideology that blood is able to wash away blood. Clearly, this was the way in which the Christian George Bush responded to 9/11. This sort of Christianity – if Christianity it is – I have no wish whatsoever to defend.